During the 1st millennium BC, they were spoken across much of Europe, in the Iberian Peninsula, from the Atlantic and North Sea coastlines, up to the Rhine valley and down the Danube valley to the Black Sea, the northern Balkan Peninsula and in central Asia Minor. The spread to Cape Breton and Patagonia occurred in modern times. Celtic languages, particularly Irish, were spoken in Australia before federation in 1901 and are still used there to some extent.[8]

The other two, Cornish and Manx, were spoken into modern times but later died as spoken community languages.[9][10][11] For both these languages, however, revitalization movements have led to the adoption of these languages by adults and children and produced some native speakers.[12][13]

Taken together, there were roughly one million native speakers of Celtic languages as of the 2000s.[14] In 2010, there were more than 1.4 million speakers of Celtic languages.[15]

Brittonic, including the living languages Breton, Cornish, and Welsh, and the extinct languages Cumbric and Pictish though Pictish may be a sister language rather than a daughter of Common Brittonic.[40] Before the arrival of Scotti on the Isle of Man in the 9th century, there may have been a Brittonic language in the Isle of Man.

Scholarly handling of the Celtic languages has been rather argumentative owing to scarceness of primary source data. Some scholars (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) distinguish Continental Celtic and Insular Celtic, arguing that the differences between the Goidelic and Brittonic languages arose after these split off from the Continental Celtic languages. Other scholars (such as Schmidt 1988) distinguish between P-Celtic and Q-Celtic, putting most of the Gaulish and Brittonic languages in the former group and the Goidelic and Celtiberian languages in the latter. The P-Celtic languages (also called Gallo-Brittonic) are sometimes seen (for example by Koch 1992) as a central innovating area as opposed to the more conservative peripheral Q-Celtic languages.

The Breton language is Brittonic, not Gaulish, though there may be some input from the latter,[41] having been introduced from Southwestern regions of Britain in the post-Roman era and having evolved into Breton – still partially intelligible by modern Welsh and Cornish speakers.

In the P/Q classification schema, the first language to split off from Proto-Celtic was Gaelic. It has characteristics that some scholars see as archaic, but others see as also being in the Brittonic languages (see Schmidt). In the Insular/Continental classification schema, the split of the former into Gaelic and Brittonic is seen as being late.

The distinction of Celtic into these four sub-families most likely occurred about 900 BC according to Gray and Atkinson[42][43] but, because of estimation uncertainty, it could be any time between 1200 and 800 BC. However, they only considered Gaelic and Brythonic. The controversial paper by Forster and Toth[44] included Gaulish and put the break-up much earlier at 3200 BC ± 1500 years. They support the Insular Celtic hypothesis. The early Celts were commonly associated with the archaeological Urnfield culture, the Hallstatt culture, and the La Tène culture, though the earlier assumption of association between language and culture is now considered to be less strong.[45][46]

There are legitimate scholarly arguments in favour of both the Insular Celtic hypothesis and the P-Celtic/Q-Celtic hypothesis. Proponents of each schema dispute the accuracy and usefulness of the other's categories. However, since the 1970s the division into Insular and Continental Celtic has become the more widely held view (Cowgill 1975; McCone 1991, 1992; Schrijver 1995), but in the middle of the 1980s, the P-Celtic/Q-Celtic hypothesis found new supporters (Lambert 1994), because of the inscription on the Larzac piece of lead (1983), the analysis of which reveals another common phonetical innovation -nm- > -nu (Gaelic ainm / Gaulish anuana, Old Welsh enuein "names"), that is less accidental than only one. The discovery of a third common innovation, would allow the specialists to come to the conclusion of a Gallo-Brittonic dialect (Schmidt 1986; Fleuriot 1986).

The interpretation of this and further evidence is still quite contested, and the main argument in favour of Insular Celtic is connected with the development of the verbal morphology and the syntax in Irish and British Celtic, which Schumacher regards as convincing, while he considers the P-Celtic/Q-Celtic division unimportant and treats Gallo-Brittonic as an outdated hypothesis.[36] Stifter affirms that the Gallo-Brittonic view is "out of favour" in the scholarly community as of 2008 and the Insular Celtic hypothesis "widely accepted".[47]

When referring only to the modern Celtic languages, since no Continental Celtic language has living descendants, "Q-Celtic" is equivalent to "Goidelic" and "P-Celtic" is equivalent to "Brittonic".

Eska (2010)[48] evaluates the evidence as supporting the following tree, based on shared innovations, though it is not always clear that the innovations are not areal features. It seems likely that Celtiberian split off before Cisalpine Celtic, but the evidence for this is not robust. On the other hand, the unity of Gaulish, Goidelic, and Brittonic is reasonably secure. Schumacher (2004, p. 86) had already cautiously considered this grouping to be likely genetic, based, among others, on the shared reformation of the sentence-initial, fully inflecting relative pronoun *i̯os, *i̯ā, *i̯od into an uninflected enclitic particle. Eska sees Cisalpine Gaulish as more akin to Lepontic than to Transalpine Gaulish.

Eska considers a division of Transalpine–Goidelic–Brittonic into Transalpine and Insular Celtic to be most probable because of the greater number of innovations in Insular Celtic than in P-Celtic, and because the Insular Celtic languages were probably not in great enough contact for those innovations to spread as part of a sprachbund. However, if they have another explanation (such as an SOV substratum language), then it is possible that P-Celtic is a valid clade, and the top branching would be:

Although there are many differences between the individual Celtic languages, they do show many family resemblances. While none of these characteristics are necessarily unique to the Celtic languages, there are few if any other languages which possess them all. They include:

Lusitanian was spoken in the area between the Douro and Tagus rivers in what is now Portugal and part of Spain. It is known only by five inscriptions, together with various place names.[49] It is an Indo-European language and some scholars have proposed that it may be a para-Celtic language that evolved alongside Celtic and is in a dialectal continuum with Tartessian and Gallaecian. This is tied to a theory of an Iberian origin for the Celtic languages.[49][50][51] Other scholars see affinities with Italic and Old European.[52][53]

Tartessian, spoken in the south-west of the Iberia Peninsula.[54] Tartessian is known by 95 inscriptions with the longest having 82 readable signs.[50][55][56] It is not known to be Indo-European and is generally left unclassified. However, John T. Koch argues that Tartessian was a Celtic language.

^Kenneth H. Jackson suggested that there were two Pictish languages, a pre-Indo-European one and a Pritenic Celtic one. This has been challenged by some scholars. See Katherine Forsyth's "Language in Pictland: the case against 'non-Indo-European Pictish'"Etext PDF (27.8 MB). See also the introduction by James & Taylor to the "Index of Celtic and Other Elements in W. J. Watson's 'The History of the Celtic Place-names of Scotland'"Etext PDF (172 KB[dead link]). Compare also the treatment of Pictish in Price's The Languages of Britain (1984) with his Languages in Britain & Ireland (2000).